Devices and methods for reading identification marks on semiconductor wafers

ABSTRACT

Methods and apparatus for machine reading of identification marks incised on semiconductor wafers. Preferred apparatus makes it possible to view the mark either as a dark image on a light background (which is in itself novel) or as a light image on a dark background. Using a television camera and an optical character reader, the viewing method can be changed automatically if a preset confidence level is not reached, and provides a signal if neither method (or the combination of the two methods) gives a satisfactory result. The method is particularly useful for successively reading the identification marks on a number of wafers stacked in a cassette. Preferred apparatus for such reading comprises a wafer support which can be pushed upwards from underneath the cassette so that the wafer to be identified rests on one ledge of the support and the adjacent wafer rests on another higher ledge on the support, thus exposing and precisely locating the identification mark on the first wafer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of our copending, commonly assignedSer. No. 07/597,082, filed Oct. 15, 1990, now U.S. Pat. No. 5,265,170which is a continuation-in-part of our commonly assigned applicationSer. No. 07/463,539 filed Jan. 11, 1990 (now abandoned).

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods and apparatus for readingidentification marks on semiconductor wafers.

2. Introduction to the Invention

It is important to be able to identify semiconductor wafers (i.e. waferscomposed of silicon or other semiconductor material such as galliumarsenide) throughout the different treatments which they undergo beforebeing diced into chips. For this purpose, it is conventional for eachwafer to carry an identification mark (usually a unique alpha-numericmark) at a standard location on one of its principal surfaces, usuallyadjacent a flat, notch or other identifiable discontinuity formed on theperiphery of the wafer so that the orientation of the wafer can beeasily determined. Many of the treatments undergone by the wafer afterit has been marked involve the deposition of metals or chemicals overthe mark. The mark must, therefore, be made by a method which deformsthe smooth surface of the wafer, e.g. by means of a laser or a diamondstylus, so that the mark will show through such deposited layers. Theterm "incised" is used in this specification to denote a mark made byany such method. The marks are very small in size, such thatmagnification is a practical requirement for reading them, and eventhough the marks are incised on the wafer, they become more and moredifficult to read as the wafer is processed. As a result, thesemiconductor industry has not been able to develop a reliable methodfor machine reading identification marks on wafers. Such a method ishighly desirable in order to achieve the objectives of reducing themisidentification of wafers, increasing production rates, increasingautomation, and minimizing handling of the wafers.

SUMMARY OF THE INVENTION

As further described in detail below, we have discovered a number ofnovel ways for improving the ease and reliability with whichidentification marks on wafers can be read. These novel expedients canbe used, individually or in combination, to achieve some or all of theobjectives set out above. The preferred methods and apparatus of thepresent invention provide, for the first time, a practical method formachine reading the identification marks on semiconductor wafers whilethe wafers are stacked in the cassettes which are conventionally used tostore and transport the wafers between processing steps.

Known methods for reading identification marks on semiconductor wafersinvolve viewing the mark as a light image on a dark background whenviewed by the human eye or by a television camera in its normal(positive) mode. Alternatively, by operating the television camera inits negative mode, the same image can be viewed as a dark image on alight background. In these known methods, the illumination source andthe viewing device are arranged so that the light which enters theviewing device is light which has been reflected from the surfaces ofthe incised mark which lie at an appropriate angle to the illuminationsource. The surfaces of the wafer which are coplanar with the principalplane of the wafer (i.e. the plane which lies between and is parallel tothe two principal surfaces of the wafer), and the surfaces of theincised mark which are not at an appropriate angle to the illuminationsource, reflect little or no light into the viewing device. The markappears, therefore, (when viewed positively) as a light image on a darkbackground. We have realized that on many wafers, particularly as thenumber of wafer treatments increases, the mark is clearer if it isviewed as an image which, if it is viewed positively (i.e. as a positiveimage) is a dark image on a light background. In this case, theillumination and the viewing device are arranged so that light reflectedfrom the coplanar surfaces of the wafer does enter the viewing device,and the light which is reflected from the non-coplanar surfaces of theincised mark does not enter the viewing device.

In its first aspect, therefore, this invention provides a method ofreading an identification mark incised on a principal surface of asemiconductor wafer which comprises using an illumination system and avideo (or television) camera or other viewing device to view theidentification mark as an image which, if it is viewed positively, is adark image on a light background. In its second aspect, this inventionprovides apparatus for carrying out such a method which comprises

(1) a wafer support for supporting a semiconductor wafer in a viewingposition;

(2) a viewing device; and

(3) a first optical system which can illuminate a target area of a wafersupported by the wafer support so that an identification mark incised onthe target area is viewed by the viewing device as an image which, if itis viewed positively, is a dark image on a light background.

We have also realized that by providing apparatus which can be used toview the identification mark either (1) as an image which, if viewedpositively, is a light image on a dark background or (2) as an imagewhich, if viewed positively, is a dark image on a light background, itis possible easily to select the most effective viewing method for anyparticular mark as a whole or for any part of a particular mark. Thus,the apparatus defined above preferably includes

(4) a second optical system which can illuminate the same target area ofa wafer supported by the wafer support so that an identification markincised on the target area is viewed by the viewing device as an imagewhich, if viewed positively, is a light image on a dark background.

The methods and apparatus of the invention are preferably operated sothat the image of the identification mark is viewed positively, and theinvention will be described chiefly by such operation. However, theinvention includes also operation so that the image is viewed as anegative image. The apparatus can be equipped with a manual switch forswitching on one or the other of the two viewing systems. Alternativelyor additionally, the image can be viewed by an optical character reader(OCR) which can be set to reject an image which does not reach apredetermined confidence level. Such an apparatus can be arranged toswitch automatically from one viewing system to the other if theconfidence level is not reached (including reading one or more of thecharacters by one system and the other characters by the other system),to proceed if the confidence level is reached in the alternative system,and to stop if the confidence level is not reached in either system (ora combination of both systems).

Any apparatus for viewing the mark alternatively as (1) an image which,when viewed positively, is a dark image or (2) an image which, whenviewed positively, is a light image, could be used to gain benefit fromthe novel concept of using the better of the two images. For example, atone extreme, there could be two completely separate optical systems,each associated with a separate viewing device, with the wafer being ina first position for one system and in a second position for the othersystem. As a practical matter, however, it is desirable to reduce thenumber of components as far as possible and for the wafer to be in thesame viewing position in both systems. Accordingly, in preferredapparatus, the first optical system includes a first source of light,the second optical system includes a second source of light which isspaced apart from the first source of light, and the target area of awafer supported by the wafer support can be viewed by the viewing devicefrom the same viewpoint by operating either the first optical system orthe second optical system. This preferred apparatus is one in which thefirst optical system, when the first light is on,

(i) causes light from the first light source to strike the target areaof a wafer supported by the wafer support,

(ii) causes light from the first light source reflected from surfaces ofthe target area which are coplanar with the principal plane of the waferto enter the viewing device, and,

(iii) causes light from the first light source reflected by anidentification mark incised on the target area not to enter the viewingdevice;

and in which the second optical system, when the second light source ison,

(i) causes light from the second source to strike the target area of awafer supported by the wafer support,

(ii) causes light from the second light source reflected from surfacesof the target area which are coplanar with the principal plane not toenter the viewing device, and

(iii) causes light from the second light source reflected by anidentification mark incised on the target area to enter the viewingdevice.

Any optical system can be used to provide the desired image to theviewing device. Preferably, for ease of reading, the image is a directimage, but a mirror image can be used with appropriate recognitiontechniques. We have obtained excellent results, when using apparatushaving two light sources as described above, through the use ofapparatus which comprises

a first mirror at an angle of about 45° to the principal plane of awafer supported by the wafer support, and

a second mirror;

the light sources and the mirrors being placed so that

(i) when the first light source is on and a wafer is supported by thewafer support, light from the first light source strikes the firstmirror, is reflected from the first mirror onto the target area of thewafer, and is reflected into the viewing device if it strikes a part ofthe target area which is coplanar with the principal plane of the wafer,and

(ii) when the second light source is on and a wafer is supported by thewafer support, light from the second light source strikes the secondmirror, is reflected from the second mirror onto the first mirror, isreflected from the first mirror onto the target area of the wafer, andis not reflected into the viewing device if it strikes a part of thetarget area which is coplanar with the principal plane of the wafer.

We have used visible light sources and corresponding viewing devices,but light of any wavelength could be used.

In addition to the improved techniques described above for reading theidentification mark on an individual wafer, we have also discovered anumber of improved techniques for successively reading theidentification marks on a plurality of wafers which are stacked in acassette in the conventional way. These techniques are useful with anyreading method, though they are particularly useful in combination withthe novel reading techniques described above.

A conventional cassette contains a plurality of uniformly spaced slots,each slot having an upper access end, side members and a partially openbottom end. The wafers are stacked in the slots, each wafer generallyhaving its flat, notch or other peripheral discontinuity at the bottomend or, less commonly, at the top end. The techniques which we havediscovered are useful for cassettes containing wafers whoseidentification marks are at the bottom ends of the slots or which can berotated so that their identification marks are at the bottom ends of theslots. We have discovered that by making use of a wafer support which isthin enough to fit in between the first and fourth wafers (or the secondand fifth, or third and sixth, etc. wafers), and which has on it twobearing surfaces which are offset from each other both in a planecoplanar with the principal planes of the wafers and in a plane at rightangles to the planes of the wafers, it is possible to push the wafersupport into the cassette so that one of the bearing surfaces contactsone wafer and displaces it by a relatively large amount, and the otherbearing surface contacts the adjacent wafer and displaces it by arelatively small amount, so that said adjacent wafer is preciselylocated in relation to the wafer support and the identification markthereon can be viewed, making use of the space created by displacing theother wafer by a relatively large amount. After the identification markhas been read, the wafer support is retracted, moved to the next wafer,and the procedure repeated. It is of course necessary that the wafersupport should also be such that it will clear the ends of the cassettewhen reading the marks on the end wafers.

Another aspect of the invention, therefore, provides an apparatus forsuccessively reading identification marks incised on a plurality ofsemiconductor wafers, the wafers being in a cassette containing aplurality of slots; each of the slots having an upper access end, sidemembers, and a partially open bottom end; and each of the wafers beingin one of the slots and having an identification mark incised on aprincipal plane of the wafer, the mark being on the portion of the waferwhich is at the bottom end of the slot, which apparatus comprises

(1) a cassette support for supporting the cassette; and

(2) a wafer support which

(a) has a first bearing surface and a second bearing surface,

(b) is mounted adjacent to the cassette support so that when a cassetteis supported by the cassette support, the wafer support is adjacent tothe partially open bottom ends of the slots in the cassette;

(c) can be moved relative to the cassette support, along an axissubstantially at right angles to a plane which is coplanar with theprincipal planes of wafers in a cassette supported by the cassettesupport, through a plurality of potential viewing positions, each of thepotential viewing positions corresponding to one of the wafers; and

(d) in each of the potential viewing positions, can be moved relative tothe cassette support in a plane which is substantially coplanar with theprincipal planes of wafers in a cassette supported by the cassettesupport, between (a) a rest position in which the support does notcontact any of the wafers in a cassette supported by the cassettesupport, and (b) an operating position in which the wafer support cansupport a wafer in a viewing position in which a target area of thewafer bearing can be viewed, the first bearing surface and the secondbearing surface being spaced apart so that, as the wafer support ismoved from the rest position to the operating position, when a cassetteof wafers is supported by the cassette support with a first wafer in thepath of the first bearing surface and an adjacent second wafer in thepath of the second bearing surface, the second bearing surface contactsthe second wafer and the first bearing surface subsequently contacts thefirst wafer, whereby the second wafer is pushed into a remote positionand the first wafer is pushed into the viewing position.

As indicated above, the apparatus is particularly useful when theidentification marks and associated peripheral discontinuities of thewafers are at the bottom ends of the slot. If they are not, then theapparatus must include means for rotating the wafers so as to bring theidentification mark into the target area (as described for example inInternational Patent Application No. PCT/US88/01728 and U.S. Pat. No.4,892,455, the disclosures of which are incorporated herein byreference). Even when the wafers are placed in the cassette so that theperipheral discontinuities are at the bottom ends of the slot, they maynot be in precisely the desired alignment. It is, therefore, preferredthat the first bearing surface should be such that the discontinuitybecomes precisely located in relation to the first bearing surface whenthe wafer is in the viewing position. For example, if the discontinuityis a flat, the bearing surface is preferably planar, so that the waferwill, if the flat is not already parallel to the bearing surface, rotateas it is pushed into the viewing position so that the flat is parallelto the bearing surface. Similarly, when the discontinuity is a notch,the bearing surface can have a notch locator at the center thereof.

The cassette of wafers can be in any orientation, providing thatmeasures are taken to ensure that the wafers are maintained in thestacked position until displaced by the wafer support and returned tothe stacked position when the wafer support is withdrawn. Preferably thecassette is oriented so that this is achieved by the force of gravityalone. It is also preferred that when a wafer is in the viewingposition, it should contact a known surface of the cassette, so that itsposition is precisely known. Accordingly, it is preferred that thecassette support is such that, when a cassette of wafers is supported bythe cassette support, the principal planes of the wafers are inclined tothe vertical at an angle of 5° to 45°, e.g. about 15°.

Any method can be used to view the identification mark on the firstwafer which is thus exposed in the viewing position. It is preferred,however, to make use of an apparatus which comprises

(a) a viewing device,

(b) a first mirror which is mounted on the wafer support so that, whenthe wafer support is in the operating position and there is a wafersupported by the wafer support in the viewing position, the first mirrorlies in a plane which is at an angle of about 45° to the principal planeof the wafer; and

(c) a second mirror which is mounted on the wafer support so that lightwhich is reflected from the target area of the wafer into the firstmirror is reflected by the first mirror into the second mirror, and isreflected by the second mirror into the viewing device.

The first mirror is necessarily very small, in order that it can fitinto the confined space between the wafers. The precise angularrelationship between the first mirror and the wafer is, therefore, veryimportant, and the apparatus preferably includes means for changing theangle between the first mirror and a wafer in the viewing position. Itis also desirable that the apparatus should comprise a first bearingsurface adjustment means for changing the position of the first bearingsurface.

We have found that the angle of the first mirror, the location and angleof the second mirror, and the location and viewing direction of thecamera or other viewing device are preferably such that the light whichenters the viewing device leaves the target area of the wafer at anangle of 65° to 80° to the principal plane of the wafer.

BRIEF DESCRIPTION OF THE DRAWING

The invention is illustrated in the accompanying drawings, in which

FIG. 1 is a diagrammatic side view of a preferred apparatus of theinvention;

FIG. 2 is a partial diagrammatic side view of the same apparatus showingthe light path when the second light source is on;

FIG. 3 is a partial diagrammatic top view of the same apparatus showingthe light path when the second light source is on;

FIG. 4 is a diagrammatic side view of the same apparatus showing thelight path when the first light source is on; and

FIGS. 5, 6 and 7 are isometric views of the wafer support in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

A preferred apparatus of the invention will now be described byreference to the accompanying drawings. It is to be understood, however,that although this description will refer to individual features of theapparatus or methods of using the apparatus in the context of theapparatus as a whole, the present disclosure includes the use of suchindividual features in combinations other than those explicitlydisclosed, either with each other, or with features previously referredto (whether as part of the present invention or as part of the priorart), or with features known to (or suggested by this specification to)those skilled in the technologies of semiconductor wafers, mechanicalengineering, optical viewing systems (including displays) and opticalcharacter recognition (including automatic processing techniquesassociated therewith).

Referring now to the drawings, a frame 1 having a horizontal base has asloping upper surface 11. Mounted on the upper surface via an indexingmechanism 2 is a cassette support 3 which supports a cassette 4 in whichare stacked silicon wafers 41, 42, 43 . . . etc. The principal planes ofthe wafers are at an angle of about 15° to the vertical, and thecassette presses against lower end wall 31 of the cassette support. Alsomounted on the frame 1, via an elevator 5, is a wafer support 6. Thewafer support can be raised and lowered by the elevator between a lowerrest position in which it is below the wafers, and an upper operatingposition (shown). The elevator speed can be controlled in a cyclicalmanner so as to minimize the speed of the wafer support when it firstcontacts the wafers, in order to reduce the danger of damaging thewafers or creating particulate debris. Also mounted on the frame is atelevision camera 7, a second light source 8 comprising a plurality oflight emitting diodes mounted on opposite sides of the camera, and afirst light source 9 which is a white light source. Light emittingdiodes have a wavelength which we have found to be particularlyappropriate for use as the second light source.

As best shown in FIGS. 5, 6 and 7, the wafer support includes a frame 60having a first mirror 61 mounted at its upper end at an angle of about45° to the frame via a thin metal flexure member 611 which enable theangle of the mirror to be adjusted by means of screws 612 and to befixed by set screw 613. A second mirror 62 is mounted on the center partof the frame 6. Also attached to the upper end of the frame 6 is a ledge63 which provides the bearing surface for the wafer whose identificationmark is to be viewed (i.e. the first bearing surface referred to above).The height of the ledge 63 can be adjusted by means of screws 634 whichhold the ledge to the frame and which compress springs 635 surroundingguide pins 636. The upper surface of the frame 6 provides the secondbearing surface 64 for the wafer which is to be pushed out of the way sothat the first wafer can be viewed.

At least the interior surfaces of the apparatus are preferably treated,e.g. painted matt black, so as to minimize stray reflections.

We claim:
 1. Apparatus for successively reading identification marksincised on a plurality of semi-conductor wafers, the wafers being in acassette containing a plurality of parallel slots; each of the slotshaving an upper access end, side members, and a partially open bottomend; and each of the wafers being in one of the slots and having anidentification mark incised on a principal plane of the wafer, the markbeing on a target area of the wafer which is at the bottom end of theslot, which apparatus comprises(1) a cassette support for supporting thecassette thereon; and (2) a wafer support which(a) has a first bearingsurface and a second bearing surface, the second surface being offsetfrom the first bearing surface both in a plane parallel with theprincipal planes of the wafers and in a plane at right angles to theprincipal planes of the wafers, (b) is mounted adjacent to the cassettesupport so that when a cassette is supported on top of the cassettesupport, the wafer support is adjacent to the partially open bottom endsof the slots in the cassette; (c) can be moved relative to the cassettesupport, along an axis substantially at right angles to a plane which iscoplanar with the principal planes of wafers in a cassette supported bythe cassette support, through a plurality of potential viewingpositions, each of the potential viewing positions corresponding to oneof the wafers; and (d) in each of the potential viewing positions, canbe moved relative to the cassette support in a plane which issubstantially coplanar with the principal planes of wafers in a cassettesupported by the cassette support, between (a) a lower rest position inwhich the support does not contact any of the wafers in a cassettesupported by the cassette support, and (b) an upper operating positionin which the first bearing surface of the wafer support supports a firstwafer in a viewing position in which the target area of the wafer havingthe identification mark incised thereon can be viewed underneath thecassette, and the second bearing surface of the wafer support supportsan adjacent second wafer in a remote position, the first and secondbearing surfaces of the wafer support being spaced apart so that, as thewafer support is moved along an upward path from the rest position tothe operating position, when a cassette of wafers is supported on thecassette support with the first wafer in the path of the first bearingsurface and the adjacent second wafer in the path of the second bearingsurface, initially the second bearing surface contacts the second waferand subsequently the first bearing surface contacts the first wafer,whereby the second wafer is displaced by a relatively large amount intothe remote position and the first wafer is displaced by a relativelysmall amount into the viewing position, the difference between theamounts by which the first and second wafers are displaced being largeenough to permit the identification mark on the first wafer to be viewedunderneath the cassette.
 2. Apparatus according to claim 1 wherein thefirst bearing surface is such that when the first wafer has adiscontinuity along its periphery and an identification mark associatedwith the discontinuity, the discontinuity and the associatedidentification mark are precisely located in relation to the firstbearing surface when the first wafer is in the viewing position. 3.Apparatus according to claim 1 wherein the cassette support is such thatwhen a cassette of wafers is supported by the cassette support, theprincipal planes of the wafers are inclined to the horizontal at anangle such that (a) when the first wafer is pushed into the viewingposition, it leans, under the force of gravity alone, against a knownsurface of the cassette, and (b) when the wafer support is moved fromthe operating position to the rest position, each of the first andsecond wafers drops back into the bottom of its slot under the force ofgravity alone.
 4. Apparatus according to claim 1 wherein the cassettesupport is such that, when a cassette of wafers is supported by thecassette support, the principal planes of the wafers are inclined to thevertical at an angle of 5° to 45°.
 5. Apparatus according to claim 1which comprises(a) a viewing device, (b) a first mirror which is mountedon the wafer support so that, when the wafer support is in the operatingposition and there is a wafer supported by the wafer support in theviewing position, the first mirror lies in a plane which is at an angleof about 45° to the principal plane of the wafer; and (c) a secondmirror which is mounted on the wafer support so that light which isreflected from the target area of the wafer into the first mirror isreflected by the first mirror into the second mirror, and is reflectedby the second mirror into the viewing device.
 6. Apparatus according toclaim 5 which comprises a first mirror adjustment means for changing theangle between the first mirror and a wafer in the viewing position, anda first bearing surface adjustment means for changing the position ofthe first bearing surface.
 7. Apparatus according to claim 1 whichcomprises a display unit which shows the image viewed by the viewingdevice when the viewing device views the target area of a wafersupported by the wafer support.
 8. Apparatus according to claim 1 whichcomprises an optical character reader which reads the image viewed bythe viewing device.
 9. Apparatus according to claim 8 which provides asignal when the optical character reader is unable to read theidentification mark with a predetermined degree of confidence. 10.Apparatus according to claim 1 which comprises a viewing device and anoptical system which, when a cassette is supported on the cassettesupport and the wafer support is in the operating position with thefirst wafer in the viewing position and the second wafer in the remoteposition, views the identification mark incised on the target area ofthe first wafer underneath the cassette.
 11. Apparatus according toclaim 10 wherein the viewing device is a television camera. 12.Apparatus according to claim 1 which comprises a cassette which issupported on the cassette support and which contains a plurality ofwafers in slots of the cassette, each of the wafers having anidentification mark incised on a target area which is at the bottom endof the slot containing the wafer.